Forensic Chemistry

Suzanne Bell

• Publisher: CRC Press
• Year: 2022
• ISBN:
  9780429804441
• Edition: 3
• Topic: Law
• Subtopic: Criminal Law
• Index: Law

Forensic Chemistry, Third Edition, the new edition of this ground-breaking book, continues to serve as the leading forensic chemistry text on the market. Fully updated, this edition describes the latest advances in current forensic chemistry analysis and practice. New and expanded coverage includes rapid advances in forensic mass spectrometry, NMR, and novel psychoactive substances (NPSs). Topics related to seized drug analysis, toxicology, combustion and fire investigation, explosives, and firearms discharge residue are described and illustrated with case studies. The role of statistics, quality assurance/quality control, uncertainty, and metrology are integrated into all topics. More pharmacological and toxicokinetic calculations are presented and discussed. Hundreds of color figures, nearly 450 total, along with graphs, illustrations, worked example problems, and case descriptions are used to show how analytical chemistry is applied to forensic practice. Coverage offer students insight into the legal context in which forensic chemistry is conducted and introduces them to the sample types and sample matrices frequently encountered in forensic laboratories.

Section 1 Metrology and Measurement

DOI: 10.4324/9780429440915-1

Forensic chemistry is analytical chemistry, and analytical chemistry is about making measurements. The data produced by a forensic chemist is data that has consequences. Decisions are made based on this data that can impact society and lives. The responsibility of the forensic analytical chemist is to make the best measurements possible. Accordingly, that is where we will begin our journey through forensic chemistry. How do you know that your data is as good as it can be? How do you ensure that your data is reported and interpreted with all the necessary information? By applying the principles that underlie measurement science. Figure I.1 presents an overview of this section and the topics covered in the next two chapters.

Figure I.1 Overview figure for this section. Our focus will be on events and procedures that occur within the laboratory. The unifying themes are metrology, statistics, and ensuring the goodness of data.

This book focuses on the analysis of evidence once it enters the doors of the laboratory (Figure I.1). As soon as the evidence is received, a paper (and digital) trail begins that will ensure that the evidence is protected by a clear chain of custody. This means that every transfer of the evidence is documented, and a responsible person identified. Subsamples may be needed for large seizures, a topic we explore in this chapter. The next section goes into detail on sample preparation and the analytical methods. Our focus in this section is the foundation of these procedures including selection and validation of analytical methods, establishing the limits and performance of methods (figures of merit), and how we ensure methods are operating as expected (quality assurance and quality control). Integrated into any chemical analysis is evaluation, interpretation, and reporting of results. The entity that submitted the evidence needs specific, clear, and complete information. Providing it requires more than outputs and values. Sufficient information and context are essential, and this includes more than a number. We will address this using the NUSAP system.

Underlying the section topics are principles of measurement science. These concepts extend beyond chemistry and include any situation in which human beings make a measurement. Because we design instruments and equipment for this purpose, significant figures must be considered. Hopefully, you will find the treatment of this subject here less daunting that you may be used to. We will see how statistics is integrated into any measurement process and how all these factors come together to ensure the “goodness” of data which can be thought of as its pedigree.

Forensic data has consequences and laboratory results can impact lives (far right of Figure I.1). Accordingly, forensic chemists must produce good data. How do we evaluate the goodness of data? In the context of forensic chemistry, we first evaluate its utility and reliability. Does it answer the question pertinent to the issue at hand? Does it provide the information needed by the decision makers (law enforcement or the legal system)? Is the data correct and complete? We summarize these considerations based on utility and reliability. The other criteria we will use in the evaluation of data and methods are reasonable-defensible-fit-for-purpose. Suppose a blood sample is submitted for blood alcohol analysis. The method used must be reasonable, defensible to scientists and laypeople, and it must answer the question: What is the blood alcohol concentration? If it does, then the method is fit-for-purpose.

The first chapter in this section explores measurement science or metrology. Metrology is based on an understanding of making measurements and characterizing them using the appropriate tools and techniques. Key among these tools are significant figures and statistics. We will cover that in Chapter 1, and with this background, we will introduce terms such as error and other associated terms vital to metrology. You will find that definitions used in everyday conversation for terms such as accuracy, precision, error, and uncertainty are incorrect or incomplete in a metrological and analytical context. Once the section is complete, you will understand how forensic chemists produce reasonable, defensible, and reliable data. In other words, you will know what is meant by “good data” and how to generate it.

Chapter 1 Making Good Measurements

DOI: 10.4324/9780429440915-2

Chapter Overview

Forensic data has consequences for individuals and society. The measurements generated in forensic chemistry must be acquired with care and expressed properly, neither over- nor understated, and with all necessary descriptors and qualifiers. How measurements are generated and reported is critical. Understanding how measurements are made starts with significant figures. We will not go through dry rules and exercises; rather, we will explore where significant figures come from and how they are used. What a number means and how it should be interpreted involves basic statistics. We will review foundational concepts, but it is assumed that you are already familiar with the basics. If not, now is a good time to do a quick review before delving into the chapter. The chapter will conclude with a discussion of hypothesis testing, which is a useful tool to add to your measurement science toolkit.

1.1 Good Measurements and Good Numbers

Metrology is the study of measurement and producing good numbers, but how do we judge if a number is “good?” In the forensic context, we can describe goodness as a function of utility and reliability. Does the data answer, or provide the information needed to answer, the relevant question(s)? Do we trust this data? How much do we trust it? We will add to this utility/reliability criteria as we move through this and the next chapter.

It is difficult to encompass the depth and breadth of metrology, given that it spans many disciplines, trades, and industries. The topic can seem daunting even to experienced forensic and analytical chemists but fear not. As we move through this discussion, you will find that most metrological principles are familiar. What may be new is how they are integrated under the umbrella of metrology. The goal is to make good measurements and produce useful and reliable data.

To focus on metrology in forensic chemistry, we will utilize a NUSAP system concept for quantitative data presentation. While not used explicitly in forensic chemistry, its concepts are making it an ideal platform for evaluating the reliability of results [1,2,3,4,5 and 6]. NUSAP stands for Number-Units-Spread-Assessment-Pedigree and contains qualitative and quantitative criteria associated with a numerical result such as the weight of a powder or blood alcohol concentration. The NUSAP system has been used for policy decisions, such as environmental modeling and risk analysis, all areas that, like forensic science, create data upon which critical decisions depend.

Consider a net weight of a white powder reported as follows:

77.56 ± 0.31 g at the 95% confidence level

As shown in Figure 1.1, this expression can be broken down into individual components. The measurand is the quantity being measured or determined, here the weight of a powder. The number (N) is 77.56; the units (U) are grams (g), and the spread (S) is ± 0.31 g. These are the quanvtitative elements of the reported value. The spread (or estimated uncertainty) of the result could have been obtained in several ways; many will be discussed later in this chapter and revisited in Chapter 2. The Student’s t-value was used here to obtain a confidence interval, a common approach, but hardly the only one. This descriptor (95% confidence interval, or CI) is the assessm...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Dedication
• Table of Contents
• Notes to Readers and Instructors
• Acknowledgments
• Section 1 Metrology and Measurement
• Section 2 Chemical Foundations
• Section 3 Drugs and Poisons
• Section 4 Combustion Evidence
• Index